84 F. M. BALFOUR. 



minal layers and organs accord in most points with my 

 own. 



The development of the Araneina may be divided into 

 four periods : (1) the segmentation ; {2) the period from the 

 close of the segmentation up to the period when the segments 

 of the body commence to be formed ; (3) the period from the 

 commencing formation of the segments to the development of 

 the full number of limbs ; (4) the subsequent stages up to 

 the attainment of the adult form. 



In my earliest stage the segmentation was already com- 

 pleted, and the embryo was formed of a single layer of large 

 flattened cells enveloping a central mass of polygonal yolk- 

 segments. 



Each yolk- segment is formed of a number of large clear 

 somewhat oval yolk-spherules. In hardened specimens the 

 yolk-spherules become polygonal, and in ova treated with 

 hot water prior to preservation are not unfrequently broken 

 up. Amongst the yolk-segments are placed a fair number, 

 of nucleated bodies of a very characteristic appearance. 

 Each of them is formed of (1) a large, often angular, nucleus, 

 filled with deeply staining bodies (nucleoli ?). (2) Of a layer 

 of protoplasm surrounding the nucleus, prolonged into a 

 protoplasmic reticulum. The exact relation of these 

 nucleated bodies to the yolk-segments is not very easy to 

 make out, but the general tendency of my observations is to 

 show (1) that each nucleated body belongs to a yolk-sphere, 

 and (2) that it is generally placed not at the centre, but to 

 one side of a yolk-sphere. If the above conclusions are 

 correct each complete yolk-segment is a cell, and each such 

 cell consists of a normal nucleus, protoplasm, and yolk- 

 spherules. There is a special layer of protoplasm surrounding 

 the nucleus, while the remainder of the protoplasm consists 

 of a reticulum holding together the yolk -spherules. Yolk- 

 cells of this character are seen in PI. IX and X, figs. 

 10—21. 



The nuclei of the yolk-cells are probably derived by divi- 

 sion from the nuclei of the segmentation rosettes (vide Lud- 

 wig. No. 5), and it is probable that they take their origin at 

 the time when the superficial layer of protoplasm separates 

 from the yolk-columns below to form the blastoderm. 



The protoplasm of the yolk-cells undergoes rapid division, 

 as is shewn by the fact that there are often two nucleated 

 bodies close together, and sometimes two nuclei in a single 

 mass of protoplasm (fig. 10). It is probable that in some 

 cases the yolk-spheres divide at the same time as the proto- 

 plasm belonging to them ; the division of the nucleated 



